You can usually pick up a phone for free (or at least cheap) these days, but if you'd prefer to build your own, DIYer David Hunt shows off how to use a Raspberry Pi as a phone.

The build here costs $158, which means it's probably cheaper to buy a phone off the shelf. Still, this one's fully customizable to your liking. It uses a touch screen, a Raspberry Pi, a GSM/GPRS module, and a few other parts. You'd obviously want to build a case for it to actually use it, but if you're in the mood to create your own phone, this is a pretty solid way to do it.